Metal powder

ABSTRACT

The present invention provides a metallic powder which exhibits superior sintering properties in a production of multilayer ceramic capacitors, and exhibits superior dispersion characteristics in forming of conductive pastes, thereby preventing delamination. Metallic powder produced by bringing a metallic chloride gas into contact with a reducing gas in a reducing temperature range is subjected to a surface treatment by a nonionic surfactant in a wet or dry process to obtain final metallic powder.

TECHNICAL FIELD

The present invention relates to a metallic powder suitable for varioususes, such as in electrical materials such as conductive pastes, bondingmaterials for titanium, and for catalysts, and in particular, theinvention relates to a metallic powder having superior sinteringproperties and dispersion characteristics, which is specificallysuitable for conductive pastes and internal electrodes in multilayerceramic capacitors.

BACKGROUND ART

Noble metals such as silver, palladium, platinum, and gold, and basemetals such as nickel, cobalt, iron, molybdenum, and tungsten have longbeen used for electrical materials such as conductive pastes, and havespecifically been used for internal electrodes in multilayer ceramiccapacitors. Multilayer ceramic capacitors generally have a constructionsuch that ceramic dielectric layers and metallic layers used forinternal electrodes are alternately laminated, and external electrodes,which are connected to the metallic layers, are connected to both endsof the ceramic dielectric layers. Materials having a high dielectricconstant such as barium titanate, strontium titanate, and yttrium oxide,are used as primary components for forming the ceramic dielectriclayers. Powders of the above noble metals and the base metals are usedas metals forming internal electrodes. Less expensive electrical partsare recently required, and therefore research on multilayer ceramiccapacitors using the latter base metallic powders has been aggressivelypursued, and nickel powder is typical among these metals.

Multilayer ceramic capacitors are generally manufactured by thefollowing method. That is, a dielectric powder, such as barium titanate,is mixed and suspended in an organic binder, and this is then formedinto a sheet by using a doctor blade method so as to produce adielectric green sheet. On the other hand, a metallic powder forinternal electrodes is mixed with an organic compound such as an organicsolvent, a plasticizer, or an organic binder so as to produce a metallicpowder paste, and the paste is printed on the green sheet by using ascreen printing method. This is then subjected to drying, laminating,firm pressing, and heating to remove the organic component, and is thensintered at about 130° C. or more. Then, external electrodes are burnedto both ends of the ceramic dielectric layers, and a multilayer ceramiccapacitor is thereby manufactured.

In manufacturing processes for multilayer ceramic capacitors such as theabove, the volume of the metallic powder changes due to expansion andcontraction thereof during the processing to vaporize and remove theorganic component from the metallic paste and the subsequent process forsintering. Similarly, the volume of the dielectric body changes due tothe sintering. That is, the different materials of the dielectric bodyand the metallic powder are simultaneously sintered, and it is thereforeinevitable that sintering properties will differ due to changes involumes due to expansion and contraction of these materials duringsintering. As a result, the process has problems in that the laminateconstruction may be broken and crack or peeling called delamination mayoccur.

Specifically, sintering in a dielectric body comprising barium titanateas a primary component is initiated at 1000° C. or more, typically at atemperature in the range of 1200 to 1300° C. However, sintering in ametallic powder for internal electrodes is initiated at a lowertemperature than this temperature, for example, normally at atemperature in the range of 400 to 500° C. in the case of nickel powder,and as a result, the volume changes due to extreme contraction, and theportion between the internal electrode and the dielectric sheet isstrained. Thus, the difference between the initiation temperatures forsintering results in differences between sintering properties of theinternal electrode and the dielectric body, and this is therefore aprimary cause of delamination. Moreover, when sintering is suddenlyinitiated at low temperatures, volume change in the final period of thesintering is large, so that delamination readily occurs. Therefore, inmetallic powders used for internal electrodes, it is desirable that theinitiating temperature for sintering be as high as possible and thatextreme sintering does not occur.

Heretofore, various methods for solving the problems of delaminationhave been proposed. For example, Japanese Patent Application, FirstPublication, No. 157903/96 discloses a method in which a sphericalpalladium powder having a predetermined diameter is heated at atemperature in the range of 100 to 200° C. in air for 24 hours or more,and a paste is then produced by using the palladium powder. JapanesePatent Application, First Publication, No. 176602/96 discloses a methodin which a palladium powder is kneaded with an acid soluble salt, suchas alkaline salts, and an organic solvent; then the organic solvent isvaporized and removed, and the mixture of the palladium powder and theacid soluble salt is heated to 300° C. or more, and is then cooled inair; and finally, the compound is dissolved so as to yield a palladiumpowder

The above conventional methods can yield some improvements in improvingsintering properties. However, these methods complicate processing andoperations, and are not sufficient to effectively prevent delaminationeven though the methods consume large amounts of energy. On the otherhand, internal electrodes are required to be formed in thin layers andto have low electrical resistance in accordance with the trends towardminiaturization and large capacity in capacitors, and therefore powdersfor internal electrodes are required to be super-fine powders, havingnot only diameters of 1μm or less, but also diameters of 0.5μm or less.When such a powder consisting of super-fine particles is mixed with anorganic solvent, the dispersion characteristics of the powder isdeteriorated and the metallic particles agglomerate with each other. Asa result, thin layers in internal electrodes cannot be easily formed dueto an increase in the number of coarse particles, and bumps anddepressions formed on a surface of electrodes may cause short circuitingand also may result in delamination. Therefore, further improvements indispersion characteristics of metallic powders in organic solvents toform conductive pastes are desired.

Furthermore, as mentioned above, multilayer ceramic capacitors withinternal electrodes made from base metallic powders, typified by nickel,are researched according to the requirements for inexpensive electricalparts. However, at present, such metallic powders, which can inhibitdelamination and are suitable for conductive pastes, have not yet beenproduced.

DISCLOSURE OF THE INVENTION

Therefore, an object of the invention is to provide a metallic powder,typified by inexpensive base metals such as nickel, in which superiorsintering properties are exhibited during production processes formultilayer ceramic capacitors, and superior dispersion characteristicsare exhibited in forming conductive pastes, thereby avoidingdelamination. More specifically, the invention provides a metallicpowder in which the initiation temperature for sintering is highcompared to that of conventional metallic powders, and is near thesintering initiation temperature of dielectric bodies used in producingmultilayer ceramic capacitors, so that delamination is inhibited.

The inventors have performed intensive research to achieve the aboveobjects. As a result, they have made the invention based on knowledgethat the desired powder can be produced by treating surfaces of metallicpowders with a surfactant. That is, the invention provides a metallicpowder in which the surface was treated with a surfactant. The metallicpowders according to the invention are metals suitable for conductivepastes, and noble metals such as silver, palladium, platinum, and goldand the like, and base metals such as nickel, cobalt, iron molybdenum,and tungsten and the like can be applied thereto. Among these metals,the base metals are preferable since they are inexpensive, andspecifically, nickel is further preferable.

Particle properties of the metallic powder of the present invention arenot particularly limited as long as nothing interferes with their use inconductive pastes. However, according to the trends toward weightreduction and compact design of electronic products in recent years, themultilayer ceramic capacitors, as parts thereof, are required to beprogressively miniaturized. Therefore, the particle size of metallicpowder used for the internal electrodes used therein is required to beprogressively smaller. Therefore, the metallic powder according to thepresent invention normally has an average particle size of 1μm or less,and is preferably in the range of 0.01 to 1μm, more preferably consistsof fine particles having an average particle size in the range of 0.05to 0.5μm. The specific surface area of the metallic powder measured bythe BET method is preferably 1 to 20 m²/g. In addition, the particleshape of the metallic powder of the invention is preferably spherical inorder to improve sintering properties and dispersion characteristics.

The metallic powder of the invention can be produced by well-knownmethods such as vapor phase methods and liquid phase methods. Inparticular, vapor phase reducing methods in which the metallic powder isformed by contacting a metallic chloride gas with a reducing gas ispreferable method since the particle size of the metallic powderproduced can be easily controlled and spherical particles can beefficiently produced. In the vapor phase reducing method, the vaporizedmetallic chloride gas reacts with reducing gas such as hydrogen. Themetallic chloride gas can be generated by heating and vaporizing a solidmetallic chloride. In consideration of avoiding oxidation of themetallic chloride and in view of energy efficiency, an advantageousproduction method is one in which a metallic chloride gas iscontinuously generated by contacting the desired metal with a chlorinegas, and the metallic chloride gas is directly supplied to a reductionprocess, thereby being brought into contact with a reducing gas andcontinuously reducing the metallic chloride powder.

During a process for production of metallic powder in a vapor phasereducing method, metallic atoms are formed instantaneously when ametallic chloride gas contacts a reducing gas, and ultrafine particlesare formed and grow through collision and cohesion of atoms. Theparticle size of the formed metallic powder depends on conditions suchas the partial pressure and the temperature of the metallic chloride gasin the reduction process. According to the above producing process formetallic powder, as a metallic chloride gas is generated in an amountaccording to the amount of chlorine gas supplied, the amount of metallicchloride gas which is supplied to the reduction process can becontrolled by controlling the amount of chlorine gas supplied. Moreover,as the metallic chloride gas is generated by the reaction of thechlorine gas with the metal, the consumption of carrier gas can bereduced (and under production conditions, no carrier gas may benecessary) compared to methods in which a solid mass of metallicchloride is heated and vaporized to form a metallic chloride gas.Therefore, the consumption of carrier gas can be reduced, andaccordingly, energy for heating can be reduced, so that production costscan be lowered.

The partial pressure of the metallic nickel chloride gas can becontrolled in the reduction process by mixing an inert gas in themetallic chloride gas generated in the chlorination process. Thus, theparticle size of the metallic powder can be controlled by controllingthe amount of chlorine gas supplied or the partial pressure of themetallic chloride gas supplied in the reduction process, and thereforethe particle size of the metallic powder can be stable and be optionallyset.

The above-described production conditions of the metallic powder by avapor phase reducing method cannot be absolutely specified; however,when a nickel powder is produced, metallic nickel as a starting rawmaterial is preferably granular, in masses, or in plates having sizes ofabout 5 to 20 mm, and the purity thereof is preferably 99.5% or more.The metallic nickel is first reacted with chlorine gas so that a nickelchloride gas is generated. The reaction temperature is 800° C. or moreand is less than the melting point of nickel, 1453° C., in order tocontinue the reaction sufficiently. In practice, the reactiontemperature is preferably in the range of 900 to 1100° C. for commercialuse, in consideration of the reaction rate and the service life of thechlorination furnace. Then, the nickel chloride gas is directly providedto a reduction process to be brought into contact with a reducing gassuch as hydrogen gas. In this case, inert gas such as nitrogen, argon,etc., is mixed with the nickel chloride gas at 1 to 30 mole percents,and this gas mixture may be introduced into the reduction process. Thetemperature of the reducing reaction may be above the temperature atwhich the reaction progresses sufficiently to completion; however, thetemperature is preferably less than the melting point of nickel, sincehandling is facilitated if a solid metallic powder is formed, and it isin practice in the range of 900 to 1100° C. in view of economicefficiency.

In this way, the nickel powder is produced by performing the reducingreaction, and the produced nickel powder is then cooled. During thecooling, it is desirable that gas flowing at near 1000° C. in thecompleted reducing reaction be cooled quickly to about 400 to 800° C. byblowing an inert gas such as a nitrogen gas, etc., and metallic powderhaving desired particle size can thereby be obtained by preventing thegeneration of secondary particles in which primary particles of theformed nickel cohere to each other. Thereafter, the formed nickel powderis separated and recovered by, for example, one or a combination of twoor more means including a bag-filter, separation by collection in wateror oil, and magnetic separation.

The invention is a metallic powder obtained by performing a surfacetreatment using a surfactant on a metallic powder produced by a processsuch as the above. As surfactants, it is possible to use one or morekinds selected from the group consisting of cationic surfactants,anionic surfactants, amphoteric surfactants, nonionic surfactants, andfluoric surfactants, and reactive surfactants.

Specifically, primary to tertiary aliphatic amine salts, aliphaticquarternary ammonium salts, benzalkonium salts, beizethonium chlorides,pyridinium salts, imidazolium salts, etc., may be mentioned as cationicsurfactants. As anionic surfactants, fatty acid soaps; N- acylaminoacids or salts thereof; carboxylates such as polyoxyethylene alkyl ethercarboxylate; sulfonates such as alkylbenzene sulfonate, alkylnaphthalenesulfonate, dialkylsulfo succinic ester, sulfosuccinic dialkylate, andalkylsulfo acetate; sulfates such as sulfated oil, fatty alcoholsulfate, polyoxyethylene alkylether sulfate, polyoxyethylenealkylphenylether sulfate, and monoglysulfate; and phosphates such aspolyoxyethylene alkyl ether phosphate, polyoxyethylene phenyl etherphosphate, and alkyl phosphate may be mentioned.

As amphoteric surfactants, carboxy betaine type, aminocarboxylate,inidadirinium betaine, lecithin, alkylamine oxide, etc. may bementioned. As nonionic surfactants, ether types such as polyoxyethylenemono-or dialkyl ethers with carbon numbers in the alkyl group of 1 to18, polyoxyethylene sec-alcohol ether, polyoxyethylene alkyl phenylether, polyoxyethylene sterol ether, and ethers such as polyoxyethylenelanolin derivative; ether esters such as polyoxyethylene glycerol fattyacid ester, polyoxyethylen castor oil, polyoxyethylene sorbitan fattyacid ester, polyoxyethylene sorbitol fatty acid ester, polyoxyethylenefatty acid alkanolamide sulfate; ester types such as polyethylene glycolfatty acid ester, ethylene glycol fatty acid ester, fatty acidmonoglyceride, polyglyceryl fatty acid ester, sorbitan fatty acid ester,propylene glycol fatty acid ester, sucrose fatty acid ester;nitrogen-conlaining types such as fatty acid alkanolamide,polyoxyethylene fatty acid amide, and polyoxyethylene alkylamine, etc.may be mentioned.

As fluoric types of surfactants, fluoroalkyl carboxylic acid,perfluoroalkyl carboxylic acid, N- perfluoro octane sulfonyl disodiumglutamate, etc., may be mentioned. As reactive surfactants,polyoxyethylene allylglycyl nonylphenol ether, polyoxyethylene propenylphenyl ether, etc., may be mentioned.

The surfactants such as the above can be used alone or in combination ofone or more. Among these surfactants, the nonionic surfactants having anHLB (hydrophilicity lipophile balance) balance, usually in the range of3 to 20, is preferably used. The type of surfactant varies according tothe treatment method, and nonionic surfactants which sufficientlydissolve in the solvent used are preferably selected. For example, whenthe metallic powders are treated in a polar organic solvents such as anaqueous solution, alcohol, ether, or acetone, hydrophilic nonionicsurfactants having HLB values in the range of 10 to 20 are preferablyused. When the metallic powders are treated in an organic hydrocarbonsolvents such as hexane and heptane, lipophilic nonionic surfactants tosome extent having HLB values generally in the range of 3 to 15 arepreferably used.

Specifically, one or more types are selected from the group consistingof polyoxyethylene alkyl phenyl ethers such as nonylphenol ether,polyoxyethylene sorbitan fatty acid esters such as polyoxyethylenesorbitan monostearate, polyglycerol fatty acid esters such aspolyglycerol monostearate, and sorbitan fatty esters such as sorbitanmonostearate are preferably used.

Any surfactants mentioned above can be used so long as they do notinterfere with the characteristics of the produced multilayer ceramiccapacitors, and therefore the type of surfactant is not limited. Whenmultilayer ceramic capacitors are produced, a metallic powder and anorganic solvent are mixed, and they are subsequently heated to removethe organic component. It is preferable to use a surfactant in which thefilm formed on the surface of the metallic powder may be simultaneouslyremoved with the organic component. Therefore, the surfactantspreferably include no metallic component which may act as contaminants,and specifically nonionic surfactants are preferably used. The nonionicsurfactants are water soluble or water-insoluble, and may be soluble insolvents such as alcohol, ether, or other hydrocarbons, but other typesof surfactants may be used according to the surface treatment method.

The metallic powder of the invention is subjected to a surface treatmentusing the above-mentioned surfactants, and wet treatment or drytreatment is applied to the surface treatment. In consideration oftreatment efficiency, wet treatment is preferable. In a wet treatment,the surfactant is dissolved in water or an organic solvent, and ametallic powder is formed into a suspension therein to perform a surfacetreatment. The concentration of the surfactant with respect to thesolvent is normally in the range of 0.001 to 10% by weight, preferablyin the range of 0.01 to 1% by weight, more preferably in the range of0.01 to 0.5% by weight. The organic solvent is a liquid at roomtemperature, and as the organic solvents, for example, alcohol, ether,acetone, aliphatic hydrocarbons having a carbon number of 5 to 18; andaromatic hydrocarbons such as kerosene, light oil, toluene, and xylene;and silicone oil, etc., may be used.

The temperature of the surface treatment is not limited, but ispreferably 0 to 200° C., and is more preferably 20 to 100° C. Treatmenttime is typically from 1 minute to 100 hours, and is preferably 1 minuteto 10 hours. In the process for producing metallic powder by a vaporphase reducing method mentioned as above, the metallic powder producedis removed and recovered by separation by collection in water or oil. Inthis case, it is effective to add the above surfactant to water or tooil to perform the surface treatment in this process. The solvent usedin oil recovery is a liquid at room temperature, and as the solvent, forexample, alcohol, ether, acetone, aliphatic hydrocarbons having a carbonnumber of 5 to 18; and aromatic hydrocarbons such as kerosene, lightoil, toluene, and xylene; and silicone oil, etc., may be used.

On the other hand, in the surface treatment by the dry method, ametallic powder is brought into contact with a surfactant using nosolvent. Such methods may comprise a metallic powder and a surfactantbeing ground together using a ball mill, a vibration mill, a pin mill, atower mill, a turbo mill, and a paint shaker, etc. Alternatively, avaporized surfactant is then brought into contact with a metallicpowder.

The amount of the surfactant and the percentage thereof in the surfacetreatment of a metallic powder is not limited, and may be the amount orpercentage thereof in which a unitary film of the surfactant is formedon the surfaces of the particles of the powder. The amount of thesurfactant used per 1 kg of a metallic powder is normally 0.0001 to 10g, and is preferably 0.001 to 1 g, and is more preferably 0.01 to 0.5 g.In the surface treatment with the surfactant, it is effective foranti-agglomeration and rust resistance to add to the metallic powder aphenolic or aminic metal deactivator, typified by benzotriazole or thederivatives thereof, or well-known antioxidants. The amount of thesurfactant adsorbed to the metallic powder after the surface treatmentwith the above surfactant is 1 to 1000 ppm per metallic particle, ispreferably 1 to 500 ppm, and is more preferably 1 to 100 ppm.

In the invention, the metallic powder may be subjected to reductionprocessing before the surface treatment with the surfactant, and thesintering properties thereof can be further improved. The reductionprocessing is performed by heating the metallic powder in a reducing gassuch as hydrogen. The temperature in the reduction processing isnormally in the range of 100 to 500° C., and is preferably in the rangeof 100 to 300° C. The processing time is normally 1 minutes or more, andis preferably in the range of 1 to 60 minutes. In the process forproducing metallic powder by the vapor phase reaction, excess hydrogengas may be supplied into the reduction furnace after the metallic powderis produced in the reduction process so as to reduce the metallic powderproduced. Alternatively, hydrogen gas may be supplied for cooling andreducing the metallic powder in a process for cooling the metallicpowder produced. Impurities such as chloride, hydroxides, and oxides,which adhere to the surfaces of the metallic powder, are removed by thereduction processing, and the metallic powder is then treated with thesurfactant. Therefore, a more effective surfactant film is formed on thesurface of the metallic powder, and a metallic powder having superiorsintering properties can be produced.

Among the metallic powders which were subjected to a surface treatmentusing a surfactant, metallic powders treated in a solvent may be driedto remove the solvent. Alternatively, the metallic powders, which weresubjected to a surface treatment with a surfactant in an organicsolvent, may be used for conductive pastes, without drying, while thepowder is dispersed in an organic solvent, and this is also a preferredembodiment of the invention. When the metallic powder is dispersed in anorganic solvent, the percentage of the organic solvent is not limited.Specifically, in consideration of operation efficiency in formingconductive pastes, the organic solvent is 0.1 to 10 parts, andpreferably 0.5 to 5 parts, per 1 part of the metallic powder. That is,the dispersion of the metallic powder in the organic solvent includesnot only the condition in which the metallic powder is suspended in theexcess organic solvent, but also the non-suspended condition in whichthe organic solvent is included in the metallic powder in the aboverange, thereby having nearly a powdered appearance or a that of slightlymoistened clay. In forming a conductive paste of a metallic powder, themetallic powder is dispersed and is kneaded with organic solvents,plasticizers, organic binders, and inorganic binders, etc. In thisprocess, the conventional powders have inferior dispersioncharacteristics, so that the metallic particles agglomerate with eachother. Therefore, when the metallic powder is used for electrodes ofmultilayer ceramic capacitors, smooth layers cannot be formed thereby,which may result in short circuiting. As a result, thin layers cannot beformed. In contrast, the metallic powder of the invention has superiordispersion characteristics, and therefore, not only does agglomerationof the particles not significantly occur when the metallic powder isused in the non-suspended condition, but also dispersion characteristicsand kneading properties in the organic solvent in forming conductivepastes are further improved by using the metallic powder as it isdispersed in the organic solvent.

When the metallic powder produced as above is used, for example, forconductive pastes, particularly for multilayer ceramic capacitors,superior sintering properties are exhibited during the productionprocess, and superior dispersion characteristics are exhibited duringthe forming of conductive pastes, and therefore delamination can beprevented. Specifically, in the metallic powder of the invention, theinitiation temperature for sintering is higher than that of theconventional metallic powders, and is near the initiation temperaturefor sintering dielectric bodies to produce multilayer ceramiccapacitors. Therefore, it is difficult for delamination to occur in themetallic powder of the invention.

BRIEF EXPLANATION OF DRAWINGS

FIG. 1 is a vertical cross sectional view showing the construction of aproduction apparatus for a metallic nickel powder according to thepresent invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments for producing a nickel powder of the presentinvention will be explained hereinafter with reference to theaccompanying drawings, clarifying effects of the present invention.Production of Nickel Powder First, a process for producing a nickelpowder by using an apparatus for producing metallic powder shown in FIG.1 will be explained hereinafter.

As a chlorination process, 15 kg of a nickel powder having an averageparticle size of 5 mm as a starting raw material was charged intochlorination furnace 1 from a raw material charging tube 11 provided atthe upper end of the chlorination furnace 1, and the temperature in thefurnace was increased to 1100° C. Chlorine gas was introduced at a flowrate of 4 Nl/min into the chlorination furnace 1, and nickel chloridegas was generated by chlorinating the metallic nickel. The nickelchloride gas was then mixed with nitrogen gas. The nitrogen gas wassupplied from an inert gas supply tube 15 provided at the lower side ofthe chlorination furnace 1 and at a flow rate of 10% (molar ratio) withrespect to the amount of chlorine gas supplied. A net 16 is preferablyprovided at the bottom of the chlorination furnace 1 so as to supportthe raw material, nickel powder M.

Then, as a reduction process, the mixture of the nickel chloride gas andthe nitrogen gas was introduced into a reduction furnace 2, from anozzle 17 at a flow rate of 2.3 m/sec (1000° C. conversion), in whichthe temperature was maintained at 100° C. by a heating device 20.Simultaneously, hydrogen gas was supplied from a reducing gas supplytube 21 provided at the top of the reduction furnace 2 at a flow rate of7 Nl/min, and the nickel chloride gas was thereby reduced. When thereducing reaction between the nickel chloride gas and the hydrogen wasin progress, a flame F, such as a burning flame which would be producedby burning of a liquid fuel such as LPG and which is aimed downward, isformed at the tip of the nozzle 17.

As a cooling process after the reduction process, the nickel powder Pproduced by the reducing reaction was brought into contact with nitrogengas supplied from a cooling gas supply tube 22 provided at the lowerside of the reduction furnace 2, thereby cooling the nickel powder P.Then, as a recovery process, the gas mixture consisting of nitrogen gas,a vapor of hydrochloric acid, and nickel powder P was introduced into anoil scrubber from a recovery tube 23, and the nickel powder P wasrecovered. The recovered nickel powder P was washed using xylene, andwas dried, thereby producing nickel powder. Surface Treatment bySurfactant

EXAMPLE 1

100 cc of a nonylphenol ether solution at 0.5% by weight was poured intoa beaker, the nickel powder produced by the above producing method wasadded to the solution, and a suspension was produced. Then, thesuspension solution was agitated at room temperature for 3 minutes, andthe beaker with suspension solution was treated in an ultrasonicvibration bath for 1 minute. Then, the solid components were separatedby screening and were dried in a vacuum at a temperature of 50° C.,thereby producing the nickel powder of Example 1.

EXAMPLE 2

A nickel powder of Example 2 was produced under the same conditions asin Example 1, except that polyoxyethylene sorbitan monostearate was usedinstead of nonylphenol ether.

EXAMPLE 3

A nickel powder of Example 3 was produced under the same conditions asin Example 1, except that an acetone solution of polyoxyethylenesorbitan monostearate was used instead of nonylphenol ether.

EXAMPLE 4

A nickel powder of Example 4 was produced under the same conditions asin Example 1, except that an heptane solution of glycerinmonooleyl etherwas used instead of nonylphenol ether.

EXAMPLE 5

10 g of the nickel powder produced by the above producing method wascharged into a flask having three openings, and hydrogen gas wassupplied therein. Then, the temperature of the atmosphere in the flaskwas inreased to 200° C., and a reduction process was performed for 5minutes. Then, 100 cc of a heptane solution of polyoxyethylene sorbitanmonostearate was added into the flask, and a suspension was produced.Then, the suspension solution was agitated at room temperature for 3minutes, and the beaker with suspension solution was treated in anultrasonic vibration bath for 1 minute. Then, the solid components wereseparated by screening and were dried in a vacuum at a temperature of50° C., thereby producing a nickel powder of Example 5.

COMPARATIVE EXAMPLE

The nickel powder produced by the above production method was nottreated with a surfactant. That is, the nickel powder as recovered wasused as a comparative example.

Measurement

The nickel powders according to the above Examples and ComparativeExamples were subjected to the following measurements. The results areshown in Table 1.

1) Initiation Temperature for Sintering

1 g of the nickel powder, camphor at a concentration of 3% by weight,and acetone at a concentration of 3% by weight were mixed, and themixture was charged into a die having a diameter of 5 mm and a length of10 mm. Then, the mixture was subjected to a surface pressure of 3 tons,whereby a test piece was produced. Initiation temperature for sinteringof this test piece was measured, using a thermal expansion andcontraction behavior (diratometry) measuring device (trade name: TD-5000S, produced by Mac Science Co.), at a heating rate of 5° C./minute, in anitrogen atmosphere.

2) Average Particle Size

The nickel powder sample was photographed using an electron microscope,200 sizes of particles of the metallic nickel powder were measured fromthe photograph, and the average thereof was calculated.

3) Particle Size Distribution in Solvent

A suitable amount of the nickel powder was suspended in a terpeneol andwas dispersed by ultrasonic vibration for 3 minutes. Particle sizes ofthe nickel powder were measured, setting the sample refractive index at1.8, and using a laser beam scattering diffraction method particle sizemeasuring apparatus (trade name: LS 230, produced by Coulter Co.), andthe particle size distribution of the volume statistic was obtained. InTable 1, the lines indicated by “D90”, “D75”, “D50”, “D25”, and “D10”show that the particle size distributions on the lines were obtained atestimation particle sizes in 90%, 75%, 50%, 25%, and 10%, respectively.As can be seen from Table 1, the initiation temperatures for sinteringof the metallic powders of the Examples are very high compared to thethat of the Comparative Example. Although there is not much differencein the average particle size between the Examples and the ComparativeExample, in the particle size distributions of the nickel powder in thesolvent, those of the metallic powders of the Examples are narrower thanthat of the metallic powder of the Comparative Example. Therefore, themetallic powders of the Examples may exhibit superior sinteringproperties in the production of multilayer ceramic capacitors, and mayexhibit superior dispersion characteristics in the forming of conductivepastes, and therefore prevention of delamination may be anticipated.

TABLE 1 Comparative Example 1 Example 2 Example 3 Example 4 Example 5Example Initiation Temperature 664 651 739 557 770 418 for Sintering (°C.) Average Particle 0.49 0.48 0.49 0.49 0.48 0.48 Size (μm) ParticleSize D90 1.48 1.60 1.50 1.55 1.45 2.07 Distribution D75 1.23 1.24 1.201.25 1.19 1.54 (μm) D50 0.95 0.89 0.90 0.94 0.85 1.15 D25 0.66 0.62 0.610.65 0.58 0.85 D10 0.41 0.44 0.43 0.45 0.40 0.65

As explained above according to the metallic powders of the invention,the initianing temperatures for sintering are high compared to those ofconventional metallic powder, and are near the initiation temperaturesfor sintering of dielectric bodies used to produce multilayer ceramiccapacitors. Therefore, the invention can yield advantages such that themetallic powder of the invention exhibits superior sintering propertiesin the production of multilayer ceramic capacitors, and exhibitssuperior dispersion characteristics in the forming of conductive pastes,and therefore delamination can be prevented.

What is claimed is:
 1. A metallic nickel powder for internal electrodesin multilayer ceramic capacitors, wherein, after the metallic nickelpowder is formed: the metallic nickel powder is subjected to a reductionprocess in which the metallic nickel powder is heated in a reducing gas;and the metallic nickel powder is subsequently subjected to a surfacetreatment with a surfactant.
 2. A metallic nickel powder for internalelectrodes in multilayer ceramic capacitors according to claim 1,wherein the metallic nickel powder is produced by contacting a metallicnickel chloride gas and a reducing gas with each other.
 3. A metallicnickel powder for internal electrodes in multilayer ceramic capacitorsaccording to claim 1, wherein the surfactant is an nonionic surfactant.4. A metallic nickel powder for internal electrodes in multilayerceramic capacitors according to claim 3, wherein the nonionic surfactantis one or more nonionic surfactants selected from the group consistingof polyoxyethylene alkyl phenyl ethers, polyoxyethylene sorbitan fattyacid esters, polyglycerol fatty acid esters, and sorbitan fatty acidsesters.
 5. A metallic nickel powder for internal electrodes inmultilayer ceramic capacitors according to claim 3, wherein the nonionicsurfactant has an HLB value in the range of 3 to
 20. 6. A metallicnickel powder for internal electrodes in multilayer ceramic capacitorsaccording to claims 1, wherein the metallic nickel powder is dispersedin an organic solvent.
 7. A metallic nickel powder for internalelectrodes in multilayer ceramic capacitors according to claims 1,wherein the metallic nickel powder has an average particle size in therange of 0.01 to 1 μm.
 8. A metallic nickel powder for internalelectrodes in multilayer ceramic capacitors according to claim 1,wherein the metallic powder has a specific surface area measured by theBET method in the range of 1 to 20 m²/g.
 9. A metallic nickel powder forinternal electrodes in multilayer ceramic capacitors according to claim1, wherein the metallic powder has a spherical particle shape.
 10. Ametallic nickel powder for internal electrodes in multilayer ceramiccapacitors according to claim 1, wherein the surface treatment isperformed at a temperature in the range of 0 to 200° C. for 1 minute to10 hours.
 11. A metallic nickel powder for internal electrodes inmultilayer ceramic capacitors according to claim 1, wherein the amountof the surfactant adsorbed to the metallic powder after the surfacetreatment is 1 to 1000 ppm per particle.
 12. A metallic nickel powderfor internal electrodes in multilayer ceramic capacitors, wherein: themetallic nickel powder is produced by contacting a metallic nickelchloride gas and a reducing gas with each other; than, the metallicnickel powder is subjected to a reduction process in which the metallicnickel powder is heated in a reducing gas at a temperature of from 100°C. to 500° C.; the metallic nickel powder is subsequently subjected to asurface treatment with a surfactant; and the metallic nickel powder hasan average particle size in a range of 0.01 to 1 μm.